Stepper: Missing unstyled example in theming section

[Preethi3S]

Fixes #6758

Description

This PR adds a missing Unstyled example for the Stepper component in the theming section of the documentation.

Previously, the Stepper page did not include an unstyled demo, whereas most other components already had unstyled examples to guide developers using Tailwind CSS or custom styling.

The new example demonstrates how to use the Stepper in unstyled mode, making it consistent with the documentation of other components.

Changes

• Added unstyleddoc.js under components/doc/stepper/theming/.
• Updated StepperDemo to include UnstyledDoc in themingDocs.
• Ensured consistency with existing theming docs (StyledDoc, TailwindDoc).
• Example showcases pt (pass-through) usage for unstyled theming.

Screenshots

Notes for Reviewers

• This is a documentation-only change; no impact on core Stepper functionality.
• Aligns Stepper docs with other components that already include unstyled examples.

[github-actions]

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